In general, an exhaust passage of an internal combustion engine is provided with an exhaust purification catalyst for purifying the exhaust gas which is discharged from the internal combustion engine. As such an exhaust purification catalyst, for example, an exhaust purification catalyst which has an oxygen storage ability is used. An exhaust purification catalyst which has an oxygen storage ability can remove the unburned gas (HC, CO, etc.) or NOX etc. in the exhaust gas which flows into the exhaust purification catalyst when the stored amount of oxygen is an appropriate amount which is smaller than the maximum storable oxygen amount. That is, if exhaust gas of an air-fuel ratio which is richer than the stoichiometric air-fuel ratio (below, also called “rich air-fuel ratio”) flows into the exhaust purification catalyst, the oxygen which is stored in the exhaust purification catalyst is used to remove the unburned gas in the exhaust gas by oxidation. On the other hand, if exhaust gas of an air-fuel ratio which is leaner than the stoichiometric air-fuel ratio (below, also called “lean air-fuel ratio”) flows into the exhaust purification catalyst, the oxygen in the exhaust gas is stored in the exhaust purification catalyst. Due to this, the surface of the exhaust purification catalyst becomes an oxygen deficient state and, along with this, the NOx in the exhaust gas is removed by reduction. As a result, the exhaust purification catalyst can purify the exhaust gas regardless of the air-fuel ratio of the exhaust gas which flows into the exhaust purification catalyst so long as the stored amount of oxygen is an appropriate amount.
In this regard, an exhaust purification catalyst deteriorates the longer the time period of usage. It is known that when the exhaust purification catalyst deteriorates in this way, along with this, the maximum storable oxygen amount of the exhaust purification catalyst is reduced. For this reason, by detecting the maximum storable oxygen amount of the exhaust purification catalyst, it is possible to detect the degree of deterioration of the exhaust purification catalyst. As the method of detection of such a maximum storable oxygen amount, for example, it is known to perform active air-fuel ratio control which switches the air-fuel ratio of the exhaust gas which flows into the exhaust purification catalyst between the rich air-fuel ratio and the lean air-fuel ratio. With this method, the output of an oxygen sensor which is provided at the downstream side of the exhaust purification catalyst in the direction of flow of exhaust, which changes along with performance of active air-fuel ratio control, is used as the basis to estimate the maximum storable oxygen amount of the exhaust purification catalyst.
In particular, in the abnormality diagnosis system which is described in PLT 1, due to the active air-fuel ratio control, the target air-fuel ratio of the exhaust gas which flows into the exhaust purification catalyst is switched several times between the rich air-fuel ratio and the lean air-fuel ratio. On top of that, the maximum storable oxygen amount is measured several times, the average value of the measured values and the variation of the measured values are calculated, and the calculated average value and variation are used as the basis to estimate the maximum storable oxygen amount. According to PLT 1, due to this, it is considered possible to diagnose a catalyst for abnormality at a high precision while considering the presence of any deterioration of the air-fuel ratio which is provided at the upstream side of the oxygen sensor or exhaust purification catalyst in the direction of exhaust flow.